Asymmetric Digital Subscriber Line (ADSL) is a form of DSL, a data communications technology that enables faster data transmission over copper telephone lines than a conventional modem can provide.

The distinguishing characteristic of ADSL over xDSL is that the volume of data flow is greater in one direction than the other, i.e. it is asymmetric. Providers usually market ADSL as a service for people to connect to the Internet in a relatively passive mode: able to use the higher speed direction for the "download" from the Internet but not needing to run servers that would require bandwidth in the other direction.

There are both technical and marketing reasons why ADSL is in many places the most common type offered to home users. On the technical side, there is likely to be more crosstalk from other circuits at the DSLAM end (where the wires from many local loops are close together) than at the customer premises. Thus the upload signal is weakest at the noisiest part of the local loop, while the download signal is strongest at the noisiest part of the local loop. It therefore makes technical sense to have the DSLAM transmit at a higher bit rate than does the modem on the customer end. Since the typical home user in fact does prefer a higher download speed, the telephone companies chose to make a virtue out of necessity, hence ADSL.

For conventional ADSL, downstream rates start at 256 kbit/s and typically reach 8 Mbit/s within 1.5 km (5000 ft) of the DSLAM equipped central office or remote terminal. Upstream rates start at 64 kbit/s and typically reach 256 kbit/s but can go as high as 1024 kbit/s. The name ADSL Lite is sometimes used for the slower versions.

Note that distances are only approximations aimed at consumers of ADSL services. Signal attenuation and Signal to Noise Ratio are defining characteristics, and can vary completely independently of distance (e.g., non-copper cabling, cable diameter). Real world performance is also dependent to the line impedance, which can change dynamically either dependent on weather conditions (very common for old overhead lines) or on the number and quality of joints or junctions in a particular cable length.

A newer variant called ADSL2 provides higher downstream rates of up to 12 Mbit/s for spans of less than 2.5 km (8000 ft). Higher symbol rates and more advanced noise shaping are responsible for these increased speeds. ADSL2+, also referred to as ITU G.992.5, boosts these rates to up to 24 Mbit/s for spans of less than 1.5 km (5000 feet). ADSL2+ also offers seamless bonding options, allowing lines with higher attenuation or lower signal to noise (SNR) ratios to be bonded together to achieve theoretically the sum total of the number of lines (i.e., up to 50 Mbit/s for two lines, etc.), as well as options in power management and seamless rate adaptation - changing the data rate used without requiring to resynchronize.

Because of the relatively low data-rate (compared to optical backbone networks), ATM is an appropriate technology for multiplexing time-critical data such as digital voice with less time-critical data such as web traffic; ADSL is commonly deployed with ATM to ensure that this remains a possibility. In a triple play scenario, different ATM virtual circuits (VCs) may be allocated for different services.

More recently, network operators are increasingly moving away from ATM, and towards Ethernet-based solutions, where 802.1Q and/or VPLS offer multiplexing solutions. The main reason for this switch is cost savings and the possibility of removing the older and more expensive ATM network.

ADSL service providers may offer either static or dynamic IP addressing. Static addressing is preferable for people who may wish to connect to their office via a virtual private network, for some Internet gaming, and for those wishing to use ADSL to host a Web server.

How ADSL works

On the wire

ADSL uses two separate frequency bands. With standard ADSL, the band from 25.875 kHz to 138 kHz is used for upstream communication, while 138 kHz - 1104 kHz is used for downstream communication.

Frequency plan for ADSL

Each of these is further divided into smaller chunks of 4.3125 kHz. During initial training, the ADSL modem tests which of the available chunks have an acceptable signal-to-noise ratio. The distance from the telephone exchange, or noise on the copper wire, may introduce errors on some frequencies. By keeping the chunks small, an error on one frequency thus need not render the line unusable: the chunk will not be used, merely resulting in reduced throughput on an otherwise functional ADSL connection.

Vendors may support usage of higher frequencies as a proprietary extension to the standard. However, this requires matching vendor-supplied equipment on both ends of the line, and will likely result in crosstalk issues that affect other lines in the same bundle.

There is a direct relationship between the number of chunks available and the throughput capacity of the ADSL connection. The exact data capacity per chunk depends on the modulation method used.

A common error is to attribute the A in ADSL to the word asynchronous. The separated frequencies of ADSL used for download and upload channels operate simultaneously; they are thus synchronous.

Modulation

ADSL initially existed in two flavors (similar to VDSL), namely CAP and DMT. CAP was the de facto standard for ADSL deployments up until 1996, deployed in 90 percent of ADSL installs at the time. However, DMT was chosen for the first ITU-T ADSL standards, G.992.1 and G.992.2 (also called G.dmt and G.lite respectively). Therefore, all modern installations of ADSL are based on the DMT modulation scheme.